1. Field of the Invention
The present invention relates to a correlator (matched filter) used in de-spreading method on a receiver in code division multiple access (CDMA) for use in a spread spectrum communication system, particularly to a matched filter which can reduce a circuit scale without deteriorating a path detection sensitivity.
2. Description of the Related Art
In a CDMA system, after primary modulation of information data is performed in a transmitting unit, code modulation (secondary modulation) is performed using a sequence of codes (a sequence of transmitted codes) having a faster rate than that of the data modulation (primary modulation), so that a transmitted complex signal is generated.
In CDMA data communication in which quadrature phase shift keying (QPSK) is used as a system of modulating the information data and codes, when the information data is set to S, and the transmitted code sequence is set to C, a transmitted complex signal TX can be represented as follows:                                                         TX              =                              S                ·                C                                                                                        =                                                (                                      Si                    +                                          j                      ⁢                                              xe2x80x83                                            ⁢                      Sq                                                        )                                ·                                  (                                      Ci                    +                                          j                      ⁢                                              xe2x80x83                                            ⁢                      Cq                                                        )                                                                                                        =                                                (                                                            Si                      ·                      Ci                                        -                                          Sq                      ·                      Cq                                                        )                                +                                  j                  ⁡                                      (                                                                  Si                        ·                        Cq                                            +                                              Sq                        ·                        Ci                                                              )                                                                                                                          =                              TXi                +                                  j                  ⁢                                      xe2x80x83                                    ⁢                  TXq                                                                                        [                  Equation          ⁢                      xe2x80x83                    ⁢          1                ]            
Here, for the information data S, when an in-phase component is represented by Si, and a quadrature component is represented by Sq, the in-phase component and the quadrature component are in an orthogonal relation, and the quadrature component Sq is multiplied by an imaginary number j and represented. Similarly, for the code sequence C, when an in-phase component is represented by Ci, and a quadrature component is represented by Cq, the in-phase component and the quadrature component are in the orthogonal relation, and the quadrature component Cq is multiplied by the imaginary number j and represented.
Furthermore, when the transmission information data is taken from the transmitted complex signal, that is, the data demodulation (de-spreading) is performed on the side of the receiving unit, the received complex signal and the transmitted code sequence used in the spreading modulation need to be subjected to the complex conjugate correlating operation.
In this case, a searcher for use on the receiver side of the CDMA system has a role of synchronization capture to select a code which is complex/conjugate with the transmitted code sequence used in the spreading modulation on the transmission side, that is, an accurate received code sequence, and further to find the transmission timing of the transmitted complex signal.
A procedure of selecting the received code sequence in the searcher comprises, in the same manner as in the data demodulation, performing the complex conjugate correlating operation of the received complex signal and the received code sequence, and performing power adding operation with respect to the operation result of the in-phase component and the quadrature component.
Here, a principle of selecting the received code sequence in the searcher will be described.
A first case will be described in which a certain code sequence C* in a complex conjugate relation with the transmitted code sequence used for generating the transmitted complex signal in the transmitter is used as a received code sequence during the correlating operation of the searcher.
Assuming that a transmitted complex signal is TX, the transmitted complex signal TX is subjected to code modulation as shown in [Equation 1] in the transmitting unit, and that the transmitted complex signal TX is received as it is to form a received complex signal, a correlating operation result R1 of the code sequence C* having the complex conjugate relation with the transmitted code sequence C, and the transmitted (received) complex signal TX is represented by the following equation:                                                         R1              =                              xe2x80x83                            ⁢                              TX                ·                                  C                  *                                                                                                        =                              xe2x80x83                            ⁢                                                (                                      TXi                    +                                          j                      ⁢                                              xe2x80x83                                            ⁢                      TXq                                                        )                                ·                                  (                                      Ci                    -                                          j                      ⁢                                              xe2x80x83                                            ⁢                      Cq                                                        )                                                                                                        =                              xe2x80x83                            ⁢                                                TXi                  ·                  Ci                                +                                  TXq                  ·                  Cq                                +                                  j                  ⁡                                      (                                                                  TXq                        ·                        Ci                                            -                                              TXi                        ·                        Cq                                                              )                                                                                                          [                  Equation          ⁢                      xe2x80x83                    ⁢          2                ]            
In the above [Equation 2], the multiplying (correlating) operations of the in-phase component TXi and quadrature component TXq of the received complex signal, and the in-phase component Ci and quadrature component Cq of the received code sequence used in the searcher are independently performed. This means that four correlators have to be prepared as hardware.
Moreover, for the second stage of the above [Equation 2], when TXi+jTXq is developed according to [Equation 1], the following is obtained:                     R1        =                  xe2x80x83                ⁢                              {                                          (                                                      Si                    ·                    Ci                                    -                                      Sq                    ·                    Cq                                                  )                            +                              j                ⁡                                  (                                                            Si                      ·                      Cq                                        +                                          Sq                      ·                      Ci                                                        )                                                      }                    ·                      (                          Ci              -                              j                ⁢                                  xe2x80x83                                ⁢                Cq                                      )                                                  =                  xe2x80x83                ⁢                              (                                          Si                ·                Ci                ·                Ci                            -                              Sq                ·                Cq                ·                Ci                            +                              Si                ·                Cq                ·                Cq                            +                              Sq                ·                Ci                ·                Cq                                      )                    +                                                  xe2x80x83                ⁢                  j          ⁡                      (                                          Si                ·                Cq                ·                Ci                            +                              Sq                ·                Ci                ·                Ci                            -                              Si                ·                Ci                ·                Cq                            +                              Sq                ·                Cq                ·                Cq                                      )                              
When the multiplication of the code sequence is represented as a correlating operation result by a correlation function Rxx, the following is obtained:                                                         =                              xe2x80x83                            ⁢                                                (                                                            Si                      ·                      Rii                                        -                                          Sq                      ·                      Riq                                        +                                          Si                      ·                      Rqq                                        +                                          Sq                      ·                      Riq                                                        )                                +                                                                                                        xe2x80x83                            ⁢                              j                ⁡                                  (                                                            Si                      ·                      Riq                                        +                                          Sq                      ·                      Rii                                        -                                          Si                      ·                      Riq                                        +                                          Sq                      ·                      Rqq                                                        )                                                                                                        =                              xe2x80x83                            ⁢                                                Si                  ·                                      (                                          Rii                      +                      Rqq                                        )                                                  +                                  j                  ⁢                                      xe2x80x83                                    ⁢                                      Sq                    ·                                          (                                              Rii                        +                        Rqq                                            )                                                                                                                              [                  Equation          ⁢                      xe2x80x83                    ⁢          3                ]            
Here, the correlation function Rxx indicates the correlating operation result of a certain code sequence and another code sequence. When two affixed letters are the same, a result (auto-correlation function) of the correlating operation of the same code sequence is indicated. When the affixed letters are different, the function is classified as a result (cross-correlation function) of the correlating operation of different code sequence.
Here, for the system of codes in the CDMA system, the auto-correlation function is highest, and the cross-correlation function has a sufficiently small value as compared with the auto-correlation function. Therefore, for the sake of simplicity, the auto-correlation function is defined as 1, and the cross-correlation function is defined as 0 in the description.
According to the above-described definition, the correlating operation result R1 of the searcher obtained by [Equation 3] can be represented as follows:
R1=2xc2x7Si+j2xc2x7Sq=X+jYxe2x80x83xe2x80x83[Equation 4]
When power adding operation is performed on the operation result of the in-phase component and the quadrature component obtained by [Equation 4], the following results:
P1=|X|2+|Y|2=4xc2x7(|Si|2+|Sq|2)
For the information data Si, Sq, when data of xc2x11 is transmitted, the following result is obtained:
P1=4xc2x7(1+1)=8
This means that when the received code sequence comprises the code sequence C* having the complex conjugate relation with the transmitted code sequence C, the power adding operation P1 obtains a constant value of 8 irrespective of the content of transmission information (information data Si, Sq).
A second case will next be described in which a code sequence Cn not placed in the complex conjugate relation with the transmitted code sequence C used for generating the transmitted complex signal in the transmitter is used as the received code sequence in the correlating operation of the searcher.
In the same manner as in the first case, assuming that the transmitted complex signal is TX, the transmitted complex signal TX is subjected to the code modulation as shown in [Equation 1] in the transmitting unit, and the transmitted complex signal TX is received as it is to form a received complex signal, a correlating operation result R2 of the code sequence Cn not placed in the complex conjugate relation with the code sequence C, and the transmitted (received) complex signal TX is represented and developed by the following equation:                                                         R2              =                              xe2x80x83                            ⁢                              TX                ·                Cn                                                                                        =                              xe2x80x83                            ⁢                                                (                                      TXi                    +                                          j                      ⁢                                              xe2x80x83                                            ⁢                      T                      ⁢                                              xe2x80x83                                            ⁢                      Xq                                                        )                                ·                                  (                                      Ck                    -                                          j                      ⁢                                              xe2x80x83                                            ⁢                      Cr                                                        )                                                                                                        =                              xe2x80x83                            ⁢                                                {                                                            (                                                                        Si                          ·                          Ci                                                -                                                  Sq                          ·                          Cq                                                                    )                                        +                                          j                      ⁢                                              xe2x80x83                                            ⁢                                              (                                                                              Si                            ·                            Cq                                                    +                                                      Sq                            ·                            Ci                                                                          )                                                                              }                                ·                                  (                                      Ck                    -                                          j                      ⁢                                              xe2x80x83                                            ⁢                      Cr                                                        )                                                                                                        =                              xe2x80x83                            ⁢                                                (                                                            Si                      ·                      Rik                                        -                                          Sq                      ·                      Rkq                                        +                                          Si                      ·                      Rqr                                        +                                          Sq                      ·                      Rir                                                        )                                +                                                                                                        xe2x80x83                            ⁢                              j                ⁡                                  (                                                            Si                      ·                      Rkq                                        +                                          Sq                      ·                      Rik                                        -                                          Si                      ·                      Rir                                        +                                          Sq                      ·                      Rqr                                                        )                                                                                        [                  Equation          ⁢                      xe2x80x83                    ⁢          5                ]            
Here, since the correlation functions Rxx in [Equation 5] are all cross-correlation functions, it can be seen that Rxx=0, and R2=0.
As apparent from the power adding operation results R1, R2 in the above-described two cases, only when the transmitted code sequence C used in the transmitter and the received code sequence used in the operation of the searcher are in the complex conjugate relation, a large power adding operation result is obtained.
Specifically, by referring to the level of the power adding operation result, it can be judged whether the transmitted code sequence C on the transmission side and the received code sequence selected on the reception side agree with each other.
Moreover, when the above-described power adding operation result is observed with time, a large power adding operation result output is obtained at a certain timing. Since the appearing timing is a transmitted signal timing to which the influence of propagation in air is added, path detection can also be realized by observing a large power adding operation result output.
In an example as one measure for realizing the correlator for the searcher to perform the correlating operation of the transmitted complex signal and the received code sequence, a matched filter is used.
Here, the conventional constitution example of the matched filter used as the correlator for the searcher will be described with reference to FIG. 6. FIG. 6 is a constitution block diagram of the conventional matched filter.
The conventional matched filter is constituted of four correlating operators 101xe2x80x2, 102xe2x80x2, 103xe2x80x2, 104xe2x80x2, two adders 105xe2x80x2, 106xe2x80x2, and a power adding operator 107xe2x80x2.
Each component of the conventional matched filter will be described.
The correlating operator 101xe2x80x2 is a correlating operator (MF Ich-1 in FIG. 6) which inputs the in-phase component TXi (Mod-I in FIG. 6) of the received complex signal TX, and the in-phase component Ci of the received code sequence C* (Code-I in FIG. 6) and takes the correlation of both components to output a correlation result.
Additionally, this correlating operator 101xe2x80x2 realizes the operation of a first term (TXixc2x7Ci) in the rightmost side of the above-described [Equation 2].
The inside of the correlating operator 101xe2x80x2 is constituted of a data register 111xe2x80x2 formed of a plurality of shift registers for successively time-shifting and holding the in-phase component TXi of the received complex signal TX, a code register 112 for successively time-shifting and holding the in-phase component Ci of the received code sequence C*, a plurality of multipliers 113xe2x80x2 for performing multiplication of the data held by the data register 111xe2x80x2 and the data held by the code register 112, and an adder 114xe2x80x2 for adding multiplication results in the multipliers 113xe2x80x2.
The correlating operator 102xe2x80x2 is a correlating operator (MF Ich-2 in FIG. 6) which inputs the quadrature component TXq (Mod-Q in FIG. 6) of the received complex signal TX and the quadrature component Cq (Code-Q in FIG. 6) of the received code sequence C* and takes the correlation of both components to output a correlation result.
Additionally, this correlating operator 102xe2x80x2 realizes the operation of the second term (TXqxc2x7Cq) in the rightmost side of the above-described [Equation 2].
The correlating operator 103xe2x80x2 is a correlating operator (MF Qch-1 in FIG. 6) which inputs the quadrature component TXq (Mod-Q in FIG. 6) of the received complex signal TX and the in-phase component Ci (Code-I in FIG. 6) of the received code sequence C* and takes the correlation of both components to output a correlation result.
Additionally, this correlating operator 103xe2x80x2 realizes the operation of the third term (TXqxc2x7Ci) in the rightmost side of the above-described [Equation 2].
The correlating operator 104xe2x80x2 is a correlating operator (MF Qch-2 in FIG. 6) which inputs the in-phase component TXi (Mod-I in FIG. 6) of the received complex signal TX and the quadrature component Cq (Code-Q in FIG. 6) of the received code sequence C* and takes the correlation of both components to output a correlation result.
Additionally, this correlating operator 104xe2x80x2 realizes the operation of the fourth term (TXixc2x7Cq) in the rightmost side of the above-described [Equation 2].
Since the inside of the correlating operator 102xe2x80x2, 103xe2x80x2 or 104xe2x80x2 is similar to that of the correlating operator 101xe2x80x2, it is not shown in FIG. 6, but it is constituted of a data register for successively time-shifting and holding the received complex signal TX inputted in each correlating operator, a code register for successively time-shifting and holding the received code sequence C*, a plurality of multipliers for performing multiplication of the data held by the data register and the data held by the code register, and an adder for adding multiplication results in the multipliers.
The adder 105xe2x80x2 is an in-phase component adder for adding the correlation result from the correlating operator 101xe2x80x2 and the correlation result from the correlating operator 102xe2x80x2 to output an in-phase component correlation result.
Additionally, this adder 105xe2x80x2 realizes the adding operation of the first and second terms in the rightmost side of the above-described [Equation 2].
The adder 106xe2x80x2 is a quadrature component adder for adding the correlation result from the correlating operator 103xe2x80x2 and the correlation result from the correlating operator 104xe2x80x2 to output a quadrature component correlation result.
Additionally, this adder 106xe2x80x2 realizes the subtracting operation of the third and fourth terms in the rightmost side of the above-described [Equation 2].
The power adding operator 107 performs the power adding operation of the in-phase component correlation result from the adder 105xe2x80x2 and the quadrature component correlation result from the adder 106xe2x80x2, and outputs a final correlating operation result.
For the operation in the conventional matched filter, in each of the correlating operators 101xe2x80x2, 102xe2x80x2, 103xe2x80x2, 104xe2x80x2, the components of the received complex signal TX successively time-shifted and held in the data register 111xe2x80x2 or the like, and the components of the received code sequence C* successively time-shifted and held in the received code register 112 or the like are multiplied/operated in the multipliers 113xe2x80x2 or the like, and the multiplication results are added in the adder 114xe2x80x2 or the like, so that each correlation result is outputted.
Subsequently, the correlation results from the correlating operators 101xe2x80x2, 102xe2x80x2 are added as the in-phase components by the adder 105xe2x80x2, the correlation results from the correlating operators 103xe2x80x2, 104xe2x80x2 are added (subtracted) as the quadrature components by the adder 106xe2x80x2, the correlation results of the in-phase and quadrature components are subjected to the power adding operation by the power adding operator 107, and a final correlating operation result is outputted, so that the selection of the received code sequence and the detection of the path are performed based on the output.
When the matched filter is used in the correlator, the received complex signals and received code sequence can be accumulated for a certain time. Therefore, even when the transmission timing of the transmitter, that is, the path timing is not known, by inputting the received complex signals to the data register 111xe2x80x2, and the like while holding a certain series of codes, it can be judged in at least one series of time whether the selected received code sequence and transmitted code sequence agree with each other.
Moreover, when the selected received code sequence and transmitted code sequence agree with each other, a large power adding operation result is outputted at a certain timing, so that the path detection can be realized as described above.
In the above-described conventional matched filter, however, the received complex signals and received code sequence are held for a certain time, and the multiplication in each time series, and the addition of the multiplication results are performed, so that the hard scale is remarkably enlarged. Moreover, when the correlating operation of QPSK-modulated received complex signals is performed, four correlating operators (101xe2x80x2 to 104xe2x80x2) need to be disposed from this nature, which causes a problem that the circuit scale becomes huge.
Therefore, in order to reduce the hard scale of each of the correlating operators 101xe2x80x2 to 104xe2x80x2 in FIG. 6, a method is proposed which comprises reducing the number of bits of each multiplier 113xe2x80x2 disposed inside, and the like to decrease the number of bits of each adder 114xe2x80x2, in-phase component adder 105xe2x80x2, and quadrature component adder 106xe2x80x2, so that the hard scale is reduced.
Since the received code sequence are held in the code register 112, and the like in one bit of time series, in order to decrease the number of bits of the multipliers 113xe2x80x2, and the like, the number of bits of the data register 111xe2x80x2, and the like is decreased, so that as a result, the number of bits of the multipliers 113xe2x80x2, and the like is decreased.
Here, a matched filter constituted to reduce the circuit scale of the conventional matched filter shown in FIG. 6 will be described with reference to FIG. 7. FIG. 7 is a block diagram of the matched filter constituted to reduce the circuit scale of the conventional matched filter.
As shown in FIG. 7, the matched filter constituted to reduce the circuit scale of the conventional matched filter is provided with four correlating operators 101xe2x80x3, 102xe2x80x3, 103xe2x80x3, 104xe2x80x3, two adders 105xe2x80x3, 106xe2x80x3, and a power adding operator 107, which are constituted in the same manner as in the conventional matched filter shown in FIG. 6, and is additionally provided with characteristic portions of binary converting units 109-I, 109-Q.
Here, the binary converting unit 109 binarizes the received complex signal and outputs one bit of data to the correlating operator, the binary converting unit 109-I binarizes the in-phase component of the received complex signal, and the binary converting unit 109-Q binarizes the quadrature component of the received complex signal.
Specifically, the binary converting unit 109 quantizes the received complex signal with A/D converter, and the like, converts the signal to digital data, takes its most significant bit (MSB), or performs binarizing otherwise.
In the matched filter of FIG. 7, the received complex signal is one-bit binarized (xe2x80x9c0xe2x80x9d, xe2x80x9c1xe2x80x9d) and inputted to the correlating operator 111xe2x80x3, and the like. For example, when xe2x80x9c0xe2x80x9d as decimal xe2x88x921, or xe2x80x9c1xe2x80x9d as decimal +1 is multiplied by the received code sequence (xe2x80x9c0xe2x80x9d is defined as xe2x88x921, and xe2x80x9c1xe2x80x9d is defined as +1 in the same manner), as a result, +1 or xe2x88x921 is obtained in decimal notation.
This multiplication result can be represented as xe2x80x9c11xe2x80x9d, xe2x80x9c01xe2x80x9d in binary notation, and in order to perform the addition of multiplication results, the two-bit adders 114xe2x80x3, 105xe2x80x3, 106xe2x80x3 may be prepared as a result.
In the operation of the matched filter of FIG. 7, the components of the received complex signal TX are binarized by the binary converting unit 109 and converted to one bit of data, successively time-shifted and held in the data register 111xe2x80x2, and the like. The subsequent operation is the same as that of the conventional matched filter.
However, in the matched filter shown in FIG. 7, since the data inputted to the data register 111xe2x80x3 or the like is of one bit, each shift register constituting the data register 111xe2x80x3 or the like may be a register of one bit. Furthermore, since the data inputted to each multiplier 113xe2x80x3 or the like is of one bit, each multiplier 113xe2x80x3 or other constitution is reduced. Additionally, when the number of bits outputted from each multiplier 113xe2x80x3 or the like decreases, the number of input bits of the adders 114xe2x80x3, 105xe2x80x3, 106xe2x80x3 decreases. Therefore, the circuit scale of each adder is reduced, and as a result the entire circuit scale of the matched filter is reduced.
In the matched filter of FIG. 7, the circuit scales of the multiplier 113xe2x80x3 and adders 114xe2x80x3, 105xe2x80x3, 106xe2x80x3 are reduced by binarizing the inputted received complex signal, and as a result the entire circuit scale is reduced, but different from the correlating operation of the n-bit accuracy data register 111xe2x80x2 to the code register 112 in the conventional matched filter of FIG. 6, the correlating operation of one bit accuracy to the code register 112 is performed, which lowers the operation accuracy and which causes a problem that the sensitivities in code determination and path selection are deteriorated.
The present invention has been developed in consideration of the above-described actual circumstances, and an object thereof is to provide a correlating operation method and a matched filter which can reduce the circuit scale without deteriorating sensitivities in the selection of received code sequence and the detection of paths.
According to the present invention, there is provided a correlating operation method, comprising: performing correlating operation of code data obtained by adding or subtracting an in-phase component and a quadrature component of a received code sequence, and an in-phase component and a quadrature component of a received complex signal; and performing power adding operation of an in-phase component correlation result and a quadrature component correlation result to obtain a correlating operation output, in which a section for performing the correlating operation of the received code sequence and the received complex signal can be realized by two sections, so that circuit scale is reduced, and power consumption can be curtailed.
According to another aspect of the present invention, there is provided a correlating operation method, comprising: inputting an in-phase component and a quadrature component of a received code sequence, and ternary-converting and inputting an in-phase component and a quadrature component of a received complex signal;
performing a first correlating operation of the ternary-converted in-phase component of the received complex signal and the in-phase component of the received code sequence, a second correlating operation of the ternary-converted quadrature component of the received complex signal and the quadrature component of the received code sequence, a third correlating operation of the ternary-converted quadrature component of the received complex signal and the in-phase component of the received code sequence, and a fourth correlating operation of the ternary-converted in-phase component of the received complex signal and the quadrature component of the received code sequence;
adding results of the first and second correlating operations to output an in-phase component correlation result, and adding results of the third and fourth correlating operations to output a quadrature component correlation result; and
performing power adding operation of the in-phase component correlation result and the quadrature component correlation result to obtain a correlating operation output. The components of the received complex signal are ternary-converted, and the number of bits is reduced, before taking, shifting, accumulating, further multiplying and adding the components. Therefore, as compared with the matched filter in which the constitution of the multiplying section and multiplication result adding section during the correlating operation is reduced and the components of the received complex signal are binary-converted and taken, the sensitivities in the selection of the received code sequence and the detection of the path can be held to some degrees.
According to further aspect of the present invention, there is provided a matched filter, comprising: an adding/subtracting unit for adding or subtracting an in-phase component and a quadrature component of a received code sequence to output code data; an in-phase component multiplying unit for multiplying in-phase component data obtained by successively time-shifting the in-phase component of the received complex signal and the code data outputted from the adding/subtracting unit; a quadrature component multiplying unit for multiplying quadrature component data obtained by successively time-shifting the quadrature component of the received complex signal and the code data outputted from the adding/subtracting unit; an in-phase component adding unit for adding the output from the in-phase component multiplying unit to output an in-phase component correlation result; a quadrature component adding unit for adding the output from the quadrature component multiplying unit to output a quadrature component correlation result; and a power adding operation unit for performing power adding operation of the in-phase component correlation result and the quadrature component correlation result to output a correlating operation result, in which the section for performing the correlating operation of the received code sequence and the received complex signal can be realized by two sections, the circuit scale is reduced, and the power consumption can be curtailed.
According to still another aspect of the present invention, there is provided a matched filter, comprising: ternary-converting an in-phase component and a quadrature component of a received complex signal; performing correlating operation with code data obtained by adding or subtracting an in-phase component and a quadrature component of a received code sequence; and performing power adding operation of correlation results of the in-phase component and the quadrature component to obtain a correlating operation output, in which the section for performing the correlating operation of the received code sequence and the received complex signal can be realized by two sections. Furthermore, by ternary-converting the components of the received complex signal, and reducing the number of bits before taking the components, the sensitivities in the selection of the received code sequence and the detection of the path are held to some degrees, while the constitution of the correlating operation section can further be reduced.